Constant-sign periodic and almost periodic solutions of a system of difference equations

where I is a subset of ℤ. Our aim is to establish criteria such that the above system has a constantsign periodic and almost periodic solution (u1, u2, …, un). The above problem is also extended to that on ℤ, ui(k)=∑l∈ℤgi(k,l)fi(l,u1(l),u2(l),...,un(l)),k∈ℤ,1≤i≤n.